ABSTRACT
THE world of living things, in which evolution has occurred, is enormously complex. It contains at the present day representatives of very ancient and simple types of organism, which have persisted almost unchanged from the earliest times of which we have knowledge, and it contains others whose evolutionary history has involved very considerable changes in the quite recent past. When we study such records as we have of the paths along which evolutionary changes have proceeded, we find what at first looks like an almost inextricable jumble of different types of events. Some lines of change have led to increases in complexity, others to simplifications. Sequences of changes which at first seem to fall into a single series along one well-defined line become resolved with further knowledge into a group of interlacing paths rather than a single one. Some paths persist only a short time and then come to a stop with the extinction of the species in question. Others continue for some period towards, for instance, a greater complexity of structure, only later to have their direction reversed, and then lead to a greater simplification. Yet, in spite of this complexity of detail, almost all biologists who have professionally studied the subject come to the conclusion that it is possible to discover a pattern in evolution as a whole-a general direction in which the process has, on the whole, proceeded-so that it is not meaningless to speak of certain organisms as being more highly evolved than others. This conclusion in fact antedates even the theory of evolution itself. It was enshrined in the late mediaeval concept of the Great Ladder of Being, which was considered to be a static orderly arrangement of unchanging types of living things into a natural hierarchy which led from the simplest up to the highest,
and eventually through man further still into the realm of the angels.1
